1. Field of the Invention
The present invention relates to spectroscopic sensors and, more particularly, to a continuous breath by breath on-airway oxygen sensor.
2. Description of the Prior Art
Oxygen consumption measurement is necessary for accurate assessment of human cardiopulmonary function. Sensor requirements for measuring oxygen consumption are: (1) accuracy to measure the difference in oxygen concentration between inspired and expired gas thus giving oxygen consumption; (2) small size and light weight so the measurement can be made on the subject's airway with no discomfort: and (3) rapid response time to continuously analyze on a breath by breath basis. No prior art oxygen sensor meets all these requirements.
Oxygen sensor technologies were reviewed by P. S. Merilainen entitled "Sensors for Oxygen Analysis: Paramagnetic, Electrochemical, Polarographic, and Zirconium Oxide Technologies," Biomedical Instrumentation & Technology, 23, 6, 1989. Electrochemical cells and polarographic sensors both produce an electrical current proportional to the number of oxygen molecules which have diffused across a membrane. These sensors do not have the required last response time due to the slow diffusion process. Additionally, accuracy degrades towards the end of the cell's life, especially when exposed to high concentrations of oxygen. Accuracy is also affected by carbon dioxide, water vapor and anesthetic agents, all of which are contained in respiratory gas. Finally, these sensors are too large to attach to the subject's airway without causing discomfort. Paramagnetic sensors utilize the strong magnetic susceptibility of oxygen to determine concentration. A sample of respiratory gas and a reference gas are mixed within a homogeneous magnetic field. A pressure signal proportional to oxygen content difference between the two gases is generated when the direction of current to the coil of the magnet is reversed. This type of oxygen sensor cannot be attached to the subject's airway due to the large size and heavy mass of the required electromagnets.
Zirconium oxide sensors are constructed of a solid electrolyte tube made of zirconium oxide covered by porous Pt-electrodes on both sides. The side of the wall exposed to higher oxygen partial pressures becomes an anode, at which oxygen molecules are ionized and transported to a cathode through the wall. The voltage generated across a cell is proportional to the logarithm of the ratio between the sample gas and the reference gas. Zirconium oxide oxygen sensors cannot be mounted on the subject's airway because they require heating to 800 degrees Celsius and are a safety hazard.
Mass spectrometers spread ionized gas molecules into a spectrum according to their mass-to-charge ratios and cannot be attached to the subject's airway due to the large size and heavy mass of the required ionizing magnets and vacuum pumps, see I. E. Sodal, "The Medical Mass Spectrometer," Biomedical Instrumentation & Technology, Vol. 23, No. 6 (1989).
Raman scattering spectrometers measure re-emitted photons caused by the collision of a photon with an oxygen molecule. A photon from a high-power laser loses energy to the oxygen molecule and is re-emitted at a lower energy and frequency. The number of photons re-emitted at the oxygen Raman scattering wavelength is proportional to the number of oxygen molecules present. Raman scattering spectrometers cannot be attached to the subject's airway due to the large size and heavy mass of the required high-power laser, heat sink and thermoelectrically cooled detectors, see Westenskow et al., "Raman Scattering for Respiratory Gas Monitoring in the Operating Room: Advantages, Specifications, and Future Advances." Biomedical Instrumentation & Technology, Vol. 23, No. 6 (1989).
Frequency modulated spectroscopy as described by Bjorklund in U.S. Pat. No. 4,297,035 and dual frequency modulation spectroscopy as described by Gallagher in U.S. Pat. No. 4,765,736 are techniques for increased sensitivity to a molecular species of interest. These techniques have limitations because, until the present sensor, continuously tunable laser diodes have not been available for wavelengths shorter than 1200 nm, see Cooper et at., "Near-infrared diode lasers monitor molecular species," Laser Focus World (November 1992). Techniques to increase the path length through a small sample volume are also described, for example, in the publication by S. M. Chernin, entitled "A New Generation of Multipass Systems," SPIE Proc, Vol. 12112, pp. 99-108 (1994).